1. Field of the Invention
This invention relates to a light-receiving module, in which an optical signal is converted to an electrical signal.
2. Related Prior Art
FIG. 7 is a plan view of a conventional light-receiving module. As shown in FIG. 7, the conventional module 101 has a semiconductor light-receiving device 131, such as a photodiode, a base 111 for mounting the light-receiving device 131, a plurality of lead terminals 121 to 124, and a semiconductor electronic device 151.
The base 111 is made of a conductive material and has a mounting surface 112. The electrical isolation between the lead terminals and the base 111 are performed by an insulating glass material, which fixes and seals the lead terminals 121 to 124 to the base 111.
The semiconductor light-receiving device 131 has a light-sensing area 132 on top of the device and has a first, a second, and a third electrode. The first electrode 133 is wire-bonded to the lead terminal 121. The third electrode 135 outputs a current signal corresponding to the inputted optical signal to the electronic device 151. Also, a resistor element 136 is integrated in the light-receiving device, which connects the first electrode 131 to the second electrode 132.
The die capacitor 141, a shape of which is a board-like, has one pad on a surface facing to the base 111 and two pads 142, 143 on the other surface. The light-receiving device 131 is mounted on one of two pads 142. The second electrode 134 of the light-receiving device 131 is wire-bonded to the pads 142 mounted the light-receiving device thereon. The other pad of the die capacitor is wire-bonded to the lead terminal 122. The resistor element 136 in the light-receiving device 131 combined with the die capacitor 141 functions as a CR-filter, that is, it operates a de-coupling filter of a bias applied to the light-receiving device.
The semiconductor electronic device 51 converts the current signal from the light-receiving device 131 into a voltage signal and amplifies the voltage signal. The electronic device 151 has a plural electrode and a ground electrode wire-bonded directly to the base 111. A first electrode 152 is wire-bonded to the pad 143 of the die capacitor 141 for supplying a bias voltage Vdd to the electronic device 151. A second electrode 153 is connected to the third electrode of the light-receiving device 131 for inputting the current signal therefrom. A third electrode 154 and a fourth electrode 155 are wire-bonded to lead terminals 123 and 124, respectively, for outputting amplified signals that are complementary to each other. A fifth electrode 156 is wire-bonded to a second die capacitor 161 mounted on the base 111. A pad of a third die capacitor 162 is also connected to the lead terminal 122.
The current optical communication using an optical module shown in FIG. 7 requires a transmission speed over 2.5 Gbps. In such high-speed application, the conventional optical module can not be applicable because a bonding wire, for example connecting the light-receiving device to the electronic device, behaves as an inductance. When the electrical signal transmits such bonding wires, a loss of the signal and reflection at points where the bonding wires are connected may occur. The higher the transmission speeds, the more serious the parasitic inductance problem. Moreover, a signal leak through a bias line influences an operation of the semiconductor electronic device. Occasionally, the instability of the signal leak causes a self-oscillation of the electronic device.
One solution for the above-mentioned problem is to lower the transmission impedance of the signal line and also the bias supply line. It is effective for lowering the transmission impedance to connect devices by plural wires or to surround the signal line by lines with low impedance such as a ground line.
However, a request for a small-sized and a lightweight module is growing more and more in the optical communication. Additional electronic parts for realizing the low impedance line are difficult to be installed in such small-sized optical module. Also, it is difficult to surround the signal line by plural lines with low impedance because only a restricted space for wiring is allowed in such module.
An object of the present invention is to provide an optical module that realizes a low impedance signal line with a simple configuration.
According to a present invention, a light-receiving module comprises a base made of a conducting material, a semiconductor light-receiving device, a semiconductor electronic device and a die capacitor. The light-receiving device has a light-sensing portion and a signal-outputting electrode for outputting current signal corresponding to an optical signal entered into the light-sensing portion. The electronic device has a signal-inputting electrode and two ground electrodes located both sides of the signal-inputting electrode. The die capacitor has a primary pad and at least two auxiliary pads. The primary pad and two auxiliary pads are arranged in parallel so as to insert the primary pad therebetween. The light-receiving device is mounted on the primary pad and the outputting electrode is wire-bonded to the signal-inputting electrode of the electronic device. The electronic device is mounted on the base adjacent to the-die capacitor so as to face the signal-inputting electrode thereof to the signal-outputting electrode of the light-receiving device. The ground electrodes located both sides of the signal-inputting electrode are connected to the respective auxiliary pads of the die capacitor, and the respective auxiliary pads are wire-bonded to the base.
In the present module, the ground electrodes located both sides of the signal-inputting electrode of the electronic device are grounded to the base through the respective auxiliary pads of the die capacitor. This enable to shorten a total length of bonding wire form the ground electrode to the pad of the die capacitor and from the pad to the base as compared with the case that the ground electrode is directly connected to the base. Therefore, it is able to avoid the influence of the parasitic inductance of the bonding wire and to protect from the deformation of the bonding wire so that the bonding wire is hard to touch an undesirable portion. The present module realizes a configuration that the signal line from the light-receiving device to the electronic device is surrounded by plural ground line, which results in lowering the impedance of the signal line.
Moreover, the die capacitor preferably includes additional pad in the outer side of the auxiliary pad relative to the primary pad. The additional pad is preferably connected to a bias electrode of the electronic device. The die capacitor thus connected to the bias electrode operates as a de-coupling capacitor, which lower the impedance of the bias line.
Further aspect of the present invention, the electronic device preferably has a pair of outputting-electrode. The signals complementary to each other are output from outputting-electrodes to the respective lead terminals. The outputting electrodes faces the respective electrodes so as to shorten the bonding wire therebetween, thus suppressing the influence of the parasitic inductance of the bonding wire.